This invention relates to a microwave signal generator, and more particularly to a millimeter wave multiplier for generating odd harmonic signal output such as a tripler. The invention finds particular application in finline structures employing hybrid integrated circuits.
Several techniques are available for generating millimeter wave frequency signals. One of the techniques is the use of a frequency multiplier for creating harmonics from the lower frequency by use of a non-linear device. In the case of frequency multiplication, much of the initial signal preparation is carried out at lower frequencies to reduce power losses and to simplify necessary circuitry at the desired output frequency.
One of the possible techniques for generating a desired output frequency is through the use of a tripler technique wherein a limiter is used to symmetrically clip the input signal in order to generate odd order harmonic products. If the clipped input signal is precisely symmetric, there will be no even order harmonic products.
Referring to FIG. 1, there is shown a schematic diagram of a representative prior art multiplier 10 typically employed as a tripler. The multiplier 10 includes first and second diodes 12 and 14 disposed in anti-parallel relationship between a signal node 16 and (ground) reference node 18 and which are together operative to clip a signal applied at the diode barrier voltage. A lowpass filter 17 fed by a radio frequency signal source 20 through a source resistance 22 presents a source signal to the signal node 16. The clipped output at the signal node 16 is applied to a bandpass filter 24, the output of which is applied to a load 26.
In operation, the diodes 12 and 14 conduct when the oscillatory input voltage at node 16 exceeds the barrier height of the diode 12 or 14 which is forward biased relative to the (ground) reference node 18, thereby clipping the input waveform. At small input levels, however, only a small third order harmonic component is generated. As a consequence, the conversion loss, i.e., the ratio of output to input, will be very high at small input levels. On the other hand, at extremely high input levels, the output level of the output signal does not vary significantly with input signal levels. Consequently, the conversion loss is said to increase. Optimum conversion loss, that is, the range of minimum loss, is limited to a range between extremes of low power and high power.
A further disadvantage of a prior art tripler has been diode reliability under high drive levels. Large signal voltages can cause excessive current which can damage diodes in such a circuit. A conventional prior art tripler provides no protection from excessive current.
What is needed, therefore, is a multiplier circuit which is capable of relatively high conversion efficiency, that is, low conversion loss over a broad range of input levels, and which has a mechanism to protect against large excursions of potentially damaging voltage.